Method of forming protective coatings for molybdenum and molybdenum-base alloys



United States Patent O METHOD OF FORMING PROTECTIVE COATINGS FOR MOLYBDENUM AND MOLYBDENUM-BASE ALLOYS Norman L. Deuble, South Orange, N. .I., assignor to Climax Molybdenum Company, New York, N. Y., a corporation of Delaware No Drawing. Application June 29, 1951, Serial No. 234,465

2 Claims. (Cl. 11765) The present invention relates to a method of forming protective coatings for molybdenum and molybdenumbase alloys.

Because of its hardness and strength at elevated temperatures, molybdenum has long been regarded a promising metal for structural members which are subject to stress at high temperatures, i. e. in the range of 1500 F. to 3000 F. or even higher, and considerable work has been done in an attempt to utilize it for such purposes. Thus far, however, its use has been restricted because of the fact that molybdenum oxidizes readily if exposed to an oxidizing atmosphere at temperatures above about 700 F. At temperatures above 1200 F., the oxide volatilizes to such an extent that progressive oxidation continues indefinitely at a rapid rate.

Many oxidation resistant coatings for molybdenum have been studied in an attempt to solve the problem, but with indifferent success. One coating which has provided substantial protection against oxidation is a coating of molybdenum disilicide. However, it is extremely 'diflicult to form an adherent coating of molybdenum disilicide on molybdenum; and, prior to the present invention, the only method of producing such coatings, namely vapor deposition, required heating the molybdenum article to be coated to a temperature in excess of 2550 F. Not only was this prior process costly, but it produced very thin coatings. Moreover, it is subject to the serious defect that it required heating the molybdenum article to a temperature above the recrystallization temperature of molybdenum, with the result that it seriously impaired the strength and toughness of the molybdenum. The recrystallization temperature of molybdenum and molybdenum-base alloys varies somewhat with the composition and mechanical treatment to which the metal has been subject, but is in the general range of 1650 F. to 2800 F., and in most cases can be considered above 2000 F.

The problem of forming an adequate protective coating on molybdenum is aggravated by the fact that a continuous protective film is essential. Even microscopic pinpoint openings or cracks will permit progressive oxidation of the underlying molybdenum at temperatures above 1200 F., since the volatile oxide may escape through such openings.

Accordingly, it is the general object of this invention to provide improved methods of forming coatings which are eiiective to protect molybdenum-base alloys from oxidation at elevated temperatures. More particularly, it is the object to provide protective coatings which are efiective at temperatures in the order of 1500 F. to 1800 F., although coatings beneficial at lower temperatures and at even higher temperatures may be produced in accordance with the principles of the invention.

Another object of the invention is to provide methods of forming oxidation resistant coatings on molybdenumbase alloys at temperatures below the recrystallization temperature of the molybdenum-base alloys.

2,788,289 Patented Apr. 9, 1957 ice Another object of the invention is to provide novel methods of forming an adherent coating containing aluminum and silicon on a molybdenum-base alloy at temperatures below the recrystallization temperature ofthe underlying alloy.

Other objects and advantages of the invention will become apparent from the following specification and claims.

The expression molybdenum-base alloys, as used herein, means any alloy of molybdenum which contains at least 50% molybdenum and which oxidizes readily at elevated temperatures and includes commercially pure molybdenum.

In accordance with the present invention, protective coatings are formed by dipping the molybdenum-base article in a molten, low-melting-point alloy of aluminum. While several types of coatings have proven successful, all of the best results have been achieved with coatings containing both aluminum and silicon. Neither aluminum nor silicon alone when similarly applied provides comparable results as a coating material.

Alloys of aluminum and silicon within the range specified have melting points below 1200 F. and, therefore, must contain other ingredients which raise the melting point to a substantial degree if protection at temperatures above 1500 F. is desired. On the other hand, if the coating is to be applied by dipping in a molten bath, it is preferred that the bath, itself, have a low melting point to avoid heating the molybdenum article above the recrystallization temperature. This dilfficulty is overcome in accordance with the present invention by the fact that there is difiusion between the molybdenum in the base article and the metals of the coating which pro duces not only a good bond but also a coating containing molybdenum which raises the melting point of the coating and enhances its capacity to provide oxidation resistance. To produce these results while the article is immersed in the bath, the temperature of the molten bath should preferably be held at 1350 F. or above.

The principles of the invention and of the several coatings and methods of application may be more fully understood from the following specific examples.

The article is dipped in a molten alloy of silicon and aluminum. The temperature of the molten alloy is not critical except that high temperatures facilitate diffusion between the molybdenum and the initial coating; and the temperature is preferably maintained below the recrystallization temperature of the article to be coated. Satisfactory results have been obtained at temperatures in the order of 1350 F. to 2000 F., but a temperature in the order of 1600 F. is preferred. The molten bath may be held in a graphite crucible and should be protected from the atmosphere by any suitable expedient in order to avoid oxide or scum. The simplest method of avoiding oxidation of the bath is to provide the bath with a floating cover of flux, such as a mixture of 73% sodium chloride and 27% sodium fluoride. The article is left suspended in the molten bath for a period of time suflicient to permit the desired degree of diffusion between the molybdenum and the coating. Immersion for periods of from 15 to 30 minutes has given satisfactory results. Increases in the time of immersion increase the thickness of the coating and, in general, thicker coatings provide greater protection against oxidation.

Satisfactory results have been achieved with the above described dip method, using as the molten coating bath an alloy comprising 88% aluminum and 12% silicon. Immersion for 30 minutes at 1600 F. in that alloy produced a smooth, adherent coating approximately .002 inch thick. It had sufiicient ductility to protect a pure molybdenum article against oxidation during a limited amount of forging at 2600 F. and also prevented oxidation of a pure molybdenum article in ordinary atmosphere at 1700 F. for 340 hours. Coatings of this character have also been used to advantage to protect test piecesof molybdenum-base alloys during high temperature tensile tests, since they. have substantial ductility.

Similar coatings formed by dipping in aluminum with from to 20% silicon .give comparable results.

position of the final coating may be different at different depths, but that point has not been fully investigated.

In some cases after diffusion treatment or exposure to oxidizing atmosphere at high temperatures, the outer surface may have a thin, porous layer of oxides, which is easily scraped oif and evidently ofierslittle, if any, protection against oxidation. A dense, adherent layer of what appears to be an oxide underlies the porous layer. The underlying main body of the coating appears to contain little, if any, oxideand is very hard.

The results achieved with the coatings of the present invention are not fully understood and are difficult to explain, particularly in view of the fact that neither aluminum nor silicon alone will protect molybdenum against oxidation at 1700 F. It is believed, however, that the coating contains a complex compound of molybdenum, aluminum and silicon, and probably a number of such compounds.

An important feature of the coatings lies in the fact that they not only'protect molybdenum against oxidation but the aluminum actually reduces any molybdenum oxides which may have formed during or before the coating operation, forming alumina which apparently remainsin the coating. Thus, it is not necessary to deoxidize the surface of the molybdenum article before applying the coating. However, visible oxide films are preferably removed. This can be done by immersion in a bath of 90% KOH plus NaNOz at 750 F. followed by a wash in cold water or by grit-blasting the surface. Grit-blasting is preferred, since it provides a roughened surface.

What is claimed is:

1. The method of forming a protective coating on a molybdenum-base alloy article which includes immersing the article in a molten bath containing aluminum and silicon at a temperature substantially above the melting point of said bath and holding said article in said bath for a period of at least 15 minutes and until diifusion between the molybdenum andthe bath forms a coating at the surface of the article containing aluminum, silicon and a substantial quantity of molybdenum and having a melting point that exceedsthe temperature of the bath, the silicon content of said bath ranging from about 5% to about 20% by weight and the temperature of the bath being below 2000 F.

2. The method of forming a protective coating on a molybdenum-base alloy article which includes immersing the article in a molten bath containing from 5% to 20% silicon by weight and the balance essentially aluminum and thereafter holding said coated article at a temperature of about 1600 F. for at least 15 min'utes'to cause diffusion between the molybdenum from the base metal and the coating.

References Cited in the file of this patent UNITED STATES PATENTS 1,695,819 ONeill Dec. 18, 1928 1,698,212 Coles 'Jan. 8, 1929 1,853,370 Marshall Apr. 12, 1932 1,899,569 Howe Feb. 28, 19.33 1,981,878 Ruben Nov. 27,1934

- 2,001,017 Feussner, et al May-l4, 1935 2,096,924 Schwarzkopf Oct. 26, .1937 2,242,254 Mansfield May 20, 1941 2,282,097 Taylor 'May 5,1942 2,304,297 Anton Dec.-8, 1942 2,391,456 Hensel Dec. 25, 1945 2,437,919 Oganowski Mar. 16, 1948 2,650,903 Garrison Sept. 1,195.3 2,682,101 Whitfield June 29, .1954 2,690,409 Wainer Sept. 28, 1954 FOREIGN PATENTS 849,790 Germany "Sept. 18, 1952 

1. THE METHOD OF FORMING A PROTECTIVE COATING ON A MOLYBDENUM-BASE ALLOY ARTICLE WHICH INCLUDES IMMERSING THE ARTICLE IN A MOLTEN BATH CONTAINING ALUMINUM AND SILICON AT A TEMPERATURE SUBSTANTIALLY ABOVE THE MELTING POINT OF SAID BATH AND HOLDING SAID ARTICLE IN SAID BATH FOR A PERIOD OF AT LEAST 15 MINUTES AND UNTIL DIFFUSION BETWEEN THE MOLYBEDENUM AND THE BATH FORMS A COATING AT THE SURFACE OF THE ARTICLE CONTAINING ALUMINUM, SILICON AND A SUBSTANTIAL QUANTITY OF MOLYBDENUM AND HAVING A MELTING POINT THAT EXCEEDS THE TEMPERATURE OF THE BATH, THE SILICON CONTENT OF SAID BATH RANGING FROM ABOUT 5% TO ABOUT 20% BY WEIGHT AND THE TEMPERATURE OF THE BATH BEING BELOW 2000*F. 